1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems for processing marine seismic data and, more particularly, to mechanisms and techniques for removing multiples and noise from marine seismic data.
2. Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure under the seafloor. While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of them. Thus, providing a high-resolution image of the subsurface is an ongoing process.
Generally, a seismic source is used to generate a seismic signal which propagates into the earth, and it is at least partially reflected by various seismic reflectors in the subsurface. The reflected waves are recorded by seismic receivers. The seismic receivers may be located on the ocean bottom, close to the ocean bottom, below a surface of the water, close to the surface of the water, close to the surface of the earth, or in boreholes in the earth. The recorded seismic datasets, e.g., travel-time, may be processed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations, e.g., to generate an image of the subsurface.
Many marine datasets suffer from high levels of noise, which make the task of processing and interpretation difficult. This is more pronounced for low fold datasets. Modern single sensor high fold datasets can also exhibit high noise levels due to poor coupling and ground or mud roll. For such datasets, it can be more practical to reduce the noise level rather than to interpolate even more densely.
Further, the marine datasets are also affected by multiples. Multiples occur when seismic energy is reflected between the sea-surface and the ocean-bottom or between the sea-surface and various reflector structures of the subsurface. The concept of multiples is illustrated in FIGS. 1A-C. FIG. 1A shows a source 10 generating a seismic wave 12 that propagates downward, toward the ocean bottom 14. The seismic wave 12 further propagates in the subsurface and gets reflected at a reflector 16. Then, the reflected wave 18 (primary reflection) propagates upward, toward the sea-surface 22. A receiver 20, located in the water, is configured to record the reflected wave 18. It is noted that a primary reflection wave experiences only one reflection. An analysis of the seismic signal generated by the primary reflection provides information about the geological feature (reflector) responsible for reflecting the seismic wave 12.
However, besides the seismic waves shown in FIG. 1A, there are other waves that do not follow the paths described above and form the multiples. For example, FIG. 1B shows that a seismic wave 24 that propagates upward and gets reflected from the sea-surface 22. Then, the reflected wave 26 propagates toward the ocean-bottom 14 and again gets reflected. This process may continue with the wave getting reflected by the reflector 16 before being recorded by the receiver 20. Other paths may be imagined that are different from those shown in FIGS. 1A and 1B and form the multiples. One more example is shown in FIG. 1C in which a seismic wave 28 is reflected by other reflectors 30 and 32 and also by the sea-surface 22 before being recorded by the receiver 20. Thus, in this case, the recorded seismic wave 34 is a down-going wave while previously, the recorded wave was an up-going wave.
The seismic waves recorded by the receiver 20 in FIGS. 1B-C have a different travel-time from the source to the receiver than will the energy that follows the primary path of FIG. 1A. Also, a multiple experiences more than one reflection. Thus, these seismic waves are an undesirable source of contamination of seismic data because they tend to obscure the interpretation of data produced by the primary reflection.
It is known that the noise and multiples are most often present in the recorded seismic data. There are conventional methods for eliminating the noise or the multiples from the recorded seismic data. For example, such a method may rely on the concept of wave-field separation, i.e., separating the up-going components (wave 18 in FIG. 1A) from the down-going components (wave 34 in FIG. 1C). However, the wave-field separation results are sometimes affected by high levels of noise on a vertical component Z, while a pressure component P, is in general of good quality. It is noted that the vertical component Z is recorded by a geophone or accelerometer while the pressure component P is recorded by a hydrophone and these devices together form the receiver 20.
The Z component is used for ocean bottom seismic (OBS) processing and in general, when processing multi-component data, including multi-component streamer data. For example, Zabihi et al., (“Enhanced wavefield separation of OBC data,” 73rd EAGE conference and exhibition, expanded abstract, 2011) indicates that “Z is needed to achieve complete pre-stack wavefield separation and also to drive processes such as mirror imaging and up-down deconvolution.”
Using the Z component, three noise-attenuation strategies are possible. Noise can be removed prior to wave-field separation, during wave-field separation, or after wave-field separation. Several noise-attenuation algorithms known in the art and practiced in seismic data processing centers attempt noise-attenuation before wave-field separation (e.g. Craft, “Geophone noise attenuation and wavefield separation using multi-dimensional decomposition technique,” 70th EAGE conference and exhibition, expanded abstract G037, 2008).
The document of Zabihi et al. proposes a method for attenuating the noise during wave-field separation, by making use of the fact that separation into up- and down-going waves simplifies the recorded wave-field and allows for improved performance of signal processing algorithms.
Regarding the multiples, a method (see Lochstanov, “Suppression of sea-floor effects from multicomponent seafloor data,” 62nd EAGE conference and exhibition, expanded abstract L52, 2000), proposes an adaptive method for removal of free surface effects from the up-going wave-field. This method involves deriving a multiple model using up-down deconvolution results and then performing an adaptive subtraction of this model from the up-going wavefield. However, because both the up-going wave-field and the model are in general affected by noise, this approach does not perform noise attenuation.
Thus, it can be seen that the existing methods either remove the noise or the multiples during or pre-wave-field separation. Therefore, there is a need to have a new mechanism or method that jointly performs noise and multiple attenuation.